1. Field of Invention
The present invention relates generally to the field of command, control, communications, computer, intelligence surveillance, and reconnaissance (C4ISR) hardware and software systems and components, and in particular using spread-spectrum communications.
2. Description of Related Art
TTNT (Tactical Targeting Networking Technology) is an advanced tactical data link currently under development by Rockwell Collins Government Systems and the Advanced Technology Center. Modes supporting Low Probability of Detection (LPD) are a highly desirable addition to existing TTNT functionality. The primary challenge for an LPD receiver is to operate at extremely low signal-to-noise ratio (SNR), often well below negative 20 dB. Because an LPD system must operate at extremely low SNR, the known sequence of chips used for signal acquisition must be very long (on the order of 1 million chips) in order to produce reasonable probabilities of detection and false alarm.
In addition, LPD performance is enhanced by frequency-hopping, which limits the amount of time an LPD signal dwells on any single carrier frequency.
Critical to LPD operation is the capability of a receiver to tolerate carrier frequency error caused by Doppler shift. Extremely low SNR and short dwell times make direct estimation of carrier frequency impractical. The present invention presents a method and apparatus whereby carrier frequency error can be estimated indirectly via estimation of the error in symbol rate. This enables longer coherent integration times and thus improves LPD receiver performance.
The majority of carrier frequency error observed by an LPD receiver in a tactical environment is a result of Doppler frequency shift. The error fe in carrier frequency caused by Doppler is a function of relative velocity v, nominal (transmitted) carrier frequency fc, and the speed of light c.
fe=±vfc/c
The same holds for the error Re in symbol rate caused by Doppler, where Rs is the nominal (transmitted) symbol rate.
Re=±vRs/c
It can be shown from the preceding expressions that the error fe in carrier frequency caused by Doppler can be expressed as a function of the error Re in symbol rate caused by Doppler.{EQUATION 1}fe=Re*fc/Rs  (EQ. 1)
Thus, if the error in symbol rate can be estimated, an estimate of the error in carrier frequency may be easily computed by multiplying the symbol rate error estimate by the ratio of nominal carrier frequency to nominal symbol rate.
Because LPD systems operate at extremely low SNR, transmission lengths are often extremely long. Error in the symbol rate caused by Doppler can result in a significant shift in symbol timing over the length of a message, which complicates signal acquisition.
In the present invention, certain terms are used as appreciated by a skilled artisan. Thus “chip” is often defined as “channel bit”. A spread spectrum system, such as used by the present invention, achieves its spectral spreading using one or more techniques such as direct sequence, forward error correction, and orthogonal channel coding. Regardless of the technique used, the bits produced by the spreading are often referred to as “chips”. These chips are modulated and sent over the channel. This distinguishes the bits created by the spreading technique (“chips”) from the information bits going into the spreading technique (“bits”). Note that spread spectrum chips are not required to be binary. “Chip rate” is the rate or frequency at which the chips are transmitted. In a spread spectrum system, the chip rate is much faster than the information bit rate, thus the spectral spreading. “Chip time” is the reciprocal of the chip rate, or the duration in time of a single chip. “Multiple chip times” refers to a period of time that is equal to more than one chip time. A “known sequence” is a sequence of chips (or bits, or symbols) of which an authorized receiver has prior knowledge. The known sequence is typically sent at the beginning of a transmission. The receiver performs a search for the known sequence in order to detect the presence of a desired signal and synchronize its signal processing to it. The process of detecting the presence of a desired signal is often referred to as the signal “acquisition”. Signal acquisition precedes signal demodulation.